We previously reported that the inhibition of catalase and glutathione peroxidase activities by treatment with 3-amino-1,2,4-triazole (ATZ) and mercaptosuccinic acid evoked sustained increases in the levels of reactive oxygen species and apoptosis in rat primary hepatocytes. Apoptosis was accompanied by increased expression of BimEL, following activation of extracellular signal-regulated kinase. The aim of this study was to characterize the mechanism underlying hepatocyte apoptosis by identifying the transcription factor that induces BimEL expression. The bim promoter region was cloned into a promoterless-luc vector, and promoter activity was monitored by a luciferase assay. The luciferase activity increased in the presence of ATZ + mercaptosuccinic acid. Pretreatment with a MEK inhibitor, U0126, or an antioxidant, vitamin C, suppressed the promoter activity. Furthermore, ATZ + mercaptosuccinic acid-induced luciferase activity was attenuated by mutation of the activator protein-1 binding site in the bim promoter region. The amounts of total and phosphorylated c-Fos increased over time in the presence of ATZ + mercaptosuccinic acid, whereas the amounts of total and phosphorylated c-Jun remained unchanged. Chromatin immunoprecipitation revealed that both c-Fos and c-Jun localized to the activator protein-1-binding site in the bim promoter region. BimEL expression and hepatocyte apoptosis were suppressed by knockdown of c-Fos and c-Jun, respectively. These results indicate that increases in c-Fos following extracellular signal-regulated kinase activation are critical for BimEL upregulation and apoptosis.